Update index.go - fixes compatibility with package_seeeduino_boards_index.json

[bmitov]

Relaxed Unmarshaling of numeric size value

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[per1234]

Another occurrence of the class of package index error that motivated this PR: VSChina/azureiotdevkit_tools#61

[cmaglie]

I cannot reproduce it anymore with the current master:

$ arduino-cli config dump
board_manager:
  additional_urls:
  - http://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json
  - https://www.pjrc.com/teensy/td_156/package_teensy_index.json
  - https://m5stack.oss-cn-shenzhen.aliyuncs.com/resource/arduino/package_m5stack_index.json
  - https://raw.githubusercontent.com/VSChina/azureiotdevkit_tools/master/package_azureboard_index.json
[......]
$ arduino-cli core update-index
Sto scaricando l'indice: package_index.tar.bz2 downloaded                                                                                                                                                  
Sto scaricando l'indice: package_damellis_attiny_index.json downloaded                                                                                                                                     
Sto scaricando l'indice: package_teensy_index.json downloaded                                                                                                                                              
Sto scaricando l'indice: package_m5stack_index.json downloaded                                                                                                                                             
Sto scaricando l'indice: package_azureboard_index.json downloaded                                                                                                                                          


$ arduino-cli core search azure
ID             Versione Nome                                      
AZ3166:stm32f4 2.0.0    MXChip - Microsoft Azure IoT Developer Kit

$ cat ~/.arduino15/package_azureboard_index.json |grep 3788484
          "size": "3788484 ",
$

may we close this one as auto-solved?

[cmaglie]

Since I'm not able to reproduce it anymore, I'm going ahead and closing this PR.
Feel free to reopen if I'm wrong or if there is more information to add.

[per1234]

The behavior was changed by #1274. Platform releases with an invalid format such as whitespace in the packages[*].platforms[*].size field are now simply ignored. The user will not see them in the platform listings, nor be able to install them.